<!DOCTYPE html>
<style>
#outer {
  width: 300px;
  height: 300px;
  background-color: blue;
  position: relative;
}
#inner {
  width: 100px;
  height: 100px;
  position: absolute;
  left: 100px;
  top: 100px;
  background-color: green;
}
</style>
<div id=outer>
  <div id=inner></div>
</div>
